The present invention relates to a low noise I-Q phase-quadrature modulator. More particularly, the invention relates to a low noise I-Q phase-quadrature for applications in radio frequency signal receivers, of the type comprising a plurality of Gilbert cell input stages driven by a voltage feed line receiving respective square wave command signals in input from a local oscillator.
As is well known, in recent years there has been notable progress and changes in the world of telecommunications. In particular, the development of new integrated electronic devices on semiconductors has contributed to a large number of these changes.
Amongst these improvements is the reduction of power consumption in portable electronic equipment. This reduction of energy consumption has permitted a reduction in weight and dimensions of this portable equipment, lengthening at the same time their useful life.
All this has resulted in a large decrease in costs, something which has made this type of portable electronic equipment accessible to a large part of the population as for example mobile cellular telephones or portable computers.
The effort to reduce the size of electronic circuitry is continuing. For example, various studies are under way at present for a new type of a radio frequency receiver RF that could be very compact.
At the moment, many designers are working on receivers based on an architecture that includes a rejection-type image mixer to permit the reduction of the stringent specifications of a band-pass filter RF, downstream, therefore making it possible to have a significant decrease in the relative costs.
An architecture of rejection-type mixer of the image band of the known type has been proposed by Hatley and is represented in FIG. 1. This architecture allows the conversion to the frequency RF of a desired band, Sw(f) of the RF signal, suppressing the image band, Su(f).
A signal 20 coming from a source LO of a local oscillator is applied to a network 21 of quadrature producing two signals 22, 23 out of phase shifted the one to the other by 90xc2x0. Such signals are applied to an I-Q modulator 24. The modulator 24 produces two output signals 25, 26 which are further out of phase in order to be applied to the input of a combiner node 27 obtaining useful signals in phase and image signals in counter-phase. In the absence of errors between the two distinct signal pathways, the image signal of the band IS at intermediate frequency is totally eliminated.
In a real mixer, nevertheless, the presence of module and phase errors in gains of the two signal pathways determines an incomplete suppression of the image at intermediate frequency. For example, the rejection requisite of image of 30 dBc requires at the same time a phase error which is less than 3xc2x0 and an amplitude error which is less than 0.3 dB (3.5%).
An embodiment of the present invention includes a current I-Q modulator having structural and functional characteristics that would allow the use of an image rejection-type mixer with a very low level of introduction of noise.
The modulator can operate in reception as if there was a xe2x80x9cfusionxe2x80x9d of the input stages formed by the Gilbert cells, therefore reducing at the same time the polarization noise of each input stage.
The characteristics and the advantages of a modulator according to the invention will become clear from the description hereinbelow, of an example of embodiment reported as indication and not limitation thereto of the invention, which makes reference to the attached drawings.